2001 v70xc awd Brake pedal hiss upon let off

Chesterlion1 · Post by **Chesterlion1** » 03 Jan 2014, 09:26

I'm new to the forums. I just purchased a 2001 V70xc AWD. When bringing the car home, I heard a hissing noise coming from the brake pedal area when I let off the brake. I took it back and they fixed the problem in about 30 seconds and said if it happens again I'll need to order a new boot. The hissing noise returned so I'll need to order a new boot. It doesn't effect the braking just an annoying sound. Is this a common problem? How hard is it to install the boot? Is it a dealer only item?

Post by **jimmy57** » 03 Jan 2014, 13:06

That is the brake booster making noise. When you release brakes there should be no vacuum venting. When you apply brakes the vacuum is vented but usually so little that you have no noise (maybe noise for aggressive quick pedal depress).
It seems they are thinking it is a normal thing and the boot for booster inside car, which has a sound dampening filter piece, is bad and letting you hear noise. They are wrong.

Post by **JRL** » 03 Jan 2014, 17:18

Translation, you need a new brake booster

Chesterlion1 · Post by **Chesterlion1** » 03 Jan 2014, 17:28

I'm not a mechanic. What does replacing the brake booster entail and how much does this cost? Should I replace the boot inside also? It seems a brake booster would effect the braking of the vehicle. I'm not having any braking problems.

Post by **xHeart** » 03 Jan 2014, 17:45

Chesterlion1 wrote:I'm not a mechanic. What does replacing the brake booster entail and how much does this cost? Should I replace the boot inside also? It seems a brake booster would effect the braking of the vehicle. I'm not having any braking problems.

Would changing seal and gasket help?
For example http://www.ipdusa.com/prodtype.asp?CAT_ID=951&numRecordPosition=0&categorycrumbs=945

Post by **jimmy57** » 03 Jan 2014, 23:10

The pedal pushrod attaches to regulator valve in back end of booster. The vacuum that is directly applied to front of booster housing from manifold hose goes through a hollow center cavity and goes to that valve. When you release brakes it allows vacuum to back side of diaphragm so there is no booster action since equal vacuum on both sides is no forward action of diaphragm against plate attached to master cylinder. When you push pedal and move the valve it first stops the vacuum from going to back of diaphragm and then begins to vent the vacuum (allows atmospheric pressure to enter) so the vac on front and reduced vac on back gives a forward force to help push the master cylinder pushrod.
OK, what does this have to do it you? Your booster valve has failed and is allowing vacuum to back of diaphragm while the vent valve is not fully closed allowing the vacuum hiss you hear.
It may stay like that for a long time or it will get worse and make for codes since idle will go lean from the leak. The other thing is the valve may fail in a way that applies brakes without you touching brake pedal.
The booster is held to firewall under dash by 4 nuts on studs and the pushrod has to be disconnected from the pedal. Outside the master cylinder has to come loose and usually can be moved enough without loosening lines. Vac hose and pedal height sensor connectors have to come off. After vacuum is out of booster the clip comes off the pedal sensor and it is moved to the new booster if it is not fitted with that sensor.

Chesterlion1 · Post by **Chesterlion1** » 05 Jan 2014, 07:31

Thank you so much. Sounds like something I don't want to do myself but sounds like I need to get the booster replaced soon. I may be in touch again.

Post by **xHeart** » 05 Jan 2014, 08:08

Chesterlion1 wrote:Thank you so much. Sounds like something I don't want to do myself but sounds like I need to get the booster replaced soon. I may be in touch again.

I find it within the scope of a DIY with pre- and post- planning and to-dos. I would start with a checklist (dig in to all MVS discussions about "brakes"), and then assess if it is indeed out of your reach.

I am in awe of our abilities to adjust when faced with difficulties.

Post by **cn90** » 19 Oct 2023, 16:34

The 2004 S60 2.5 T with 102K miles runs fine, brakes fine etc.

I just noticed that:
- Engine off, radio off.
- When I press on the brake pedal, I hear a hissing noise.
- No noise when brake pedal is released.

- After the brake pedal is pressed some 10 times (no
more vacuum assist), no more hissing noise.

- With engine running, same observation: hissing noise only when brake pedal is pressed.

Is this normal from factory?
Car runs fine otherwise.

Post by **vtl** » 20 Oct 2023, 04:53

Normal.